Update arcAppsPrivate API to stable channel

This will allow apps whitelisted for this API (ie the Highlights app for
Demo Mode) to use this API in M70 stable.

Bug: 819404
Change-Id: I068684b743da01b928b2f25c326ef6166806473e
Reviewed-on: https://chromium-review.googlesource.com/1176703Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583514}

chrome/common/extensions/api/_permission_features.json

@@ -48,7 +48,7 @@
     ]
   },
   "arcAppsPrivate": {
-    "channel": "trunk",
+    "channel": "stable",
     "extension_types": ["platform_app"],
     "whitelist": [
       "46578A13607D38F1DC8E280C4F499FB0A2F9565C", // http://crbug.com/819404